For maximum efficiency, it is desirable to make a gas reacting apparatus wherein a high interfacial surface area coupled with turbulent mixing and long residence time are effected simultaneously and which, as such, is capable of removing both solute gases and particulate matter, either separately or simultaneously with high efficiency, with the former being separated via a gas-liquid, gas-liquid-solid or gas-gas-liquid reaction, depending on the application. To-date, as far as we are aware, there is currently no one type of gas reacting apparatus available that is capable of achieving high performance for all of the above criteria, due to a compromise generally being made between generation of very fine droplets for affecting very high surface area on one hand and long residence time on the other.
One of the methods for separating particulate matter in solid or slurry form from a gas stream wherein a dirty gas stream enters a conduit at one end and is moved through it by a fan at the other end and where a fine spray of liquid, preferably water, is cocurrently discharged into such a gas stream upstream from the fan is described in U.S. Pat. No. 4,067,703, issued Jan. 10, 1978, the disclosure of which is incorporated herein by reference. The patent disclosure, while showing a highly-effective method for removal of particulate matter from a gas stream, does not teach how the apparatus can be used as a gas reacting apparatus for removing solute gases. Also, in many aspects, the technique disclosed therein does not provide the absorbing and reacting environment required for effecting high removal efficiency of solute gases. For example, in the foregoing prior art patent, the mixture of gas and particulate matter enters only a single contact spray zone provided by one nozzle in which an atomized liquid spray is injected cocurrent to the dusty air stream flow. While this mode of operation as disclosed has proven to be highly effective for removing particulate matter and effecting lower pressure drop in the apparatus wherein particles were collected primarily by impaction upon the finely-divided water droplets introduced, followed by further agglomeration and impaction on the fan blades as the gas moves through the device, the residence time available for mass transfer is too short and the effective interfacial surface area and turbulence generated by a single contact spray cocurrently oriented to the gas stream are not sufficient to effect high removal efficiencies of solute gases of relatively low solubility in aqueous solution. It is, therefore, desirable to provide for an improved gas reacting apparatus and method which overcome some of the shortcomings of the foregoing prior art apparatus in which increased available residence time, interfacial surface area and turbulence are generated to result in accelerated absorption and reaction kinetics and intimate gas/liquid contact and thus, in high removal efficiency of both solute gas and particulate matter.
While high interfacial surface area, turbulent mixing and long residence time for effecting accelerated mass transfer of solute gases and effectively separating particulate matter are the major criteria in gas reacting apparatus selection, often a compromise must be made between removal efficiency on one hand and operating reliability on the other. Several other factors then also enter into consideration, such as slurry handling without plugging, turndown, and gas and liquid distribution.
The basic processes for removal of solute gases from gas streams, particularly flue gas desulfurization processes, are based on readily-available, low-cost absorbents in the form of an aqueous slurry, such as a lime or limestone slurry, or a clear aqueous solution, such as caustic or ammoniacal solutions. Various prior art methods are in use to bring the above absorbing and reacting media into intimate contact with the pollutant-laden gas. Packed bed and perforated trays, which are known to be efficient gas absorption and reaction devices, are usually the first choice for designers of flue gas desulfurization (FGD) systems, but experience has shown that they are not completely satisfactory. Both perforated trays which bubble the gas through a thin layer of liquid, and packed beds, which pass the gas over solid packing elements that are wetted with the liquid have many narrow passages which are subject to plugging especially if particulate loads are heavy, or if precipitates are formed during the chemisorption process. Such conditions can be minimized by careful process design, but the possibility of scaling under upset conditions still exists and compromises reliability. Another principal disadvantage of both of the above types of scrubbers is their extremely limited turndown capability.
Consequently, heretofore, the gas reacting devices of preference and the ones that would seem to be the answer have been the venturi or open spray tower wherein the internal complexity is low and yet where a relatively large surface area of the liquid is generated per unit volume of gas treated. While the above devices have evolved considerably over the last decade in a way to improve their performance and to remove some of their shortcomings, the current trend in the design particularly of FGD systems, is away from venturis to spray towers or combination towers. The venturi design, although capable of producing a relatively large liquid surface area for contact with the gas stream, was abandoned largely because the very short liquid/gas contact time (attributable largely to the absorbing medium being introduced cocurrently to the gas stream in the throat of the venturi) results in low sulfur dioxide removal. Also, being a relatively high energy device, it is incapable of producing an evenly distributed regime of droplets at high density unless an `overkill` situation exists wherein excess energy in the form of velocity pressure is added to the gas stream to provide for the required uniform distribution. Spray towers, on the other hand, have few internal components in the gas/liquid contact zone and the use of sprays appears to offer an easy way of increasing the surface area exposed to the gas. However, the sprays are usually introduced at the top of the spray tower and drop by gravity in counter-current flow to the gas stream. To avoid being entrained in the gas stream, the normal size of the droplets sprayed is in the order of 1000 to 2500 microns in diameter. Thus, to increase the surface area exposed to the gas phase and residence time, very high liquid to gas (L/G) ratios and large towers must be employed, all of which substantially increase the capital and operating cost requirement. To effect good gas distribution, a large number of spray nozzles must be used, so that the tower cross-section is uniformly covered with the spray pattern. However, failure of one or two nozzles usually creates a path of least resistance through which the gas can flow, thereby reducing the efficiency of the apparatus.
In addition, the large size of droplets used in spray towers reduces substantially the capability of the apparatus to efficiently remove dust particles in the low particle size range, typically less than 3 microns. With the larger droplets, the decreased gas-liquid surface area can be compensated for by increasing the tower size, the number of spray headers, and circulation rates of the scrubbing liquor, all of which increase the tower space requirement, thereby initial cost and energy consumption. Droplet entrainment and mist elimination, while rather effectively being addressed by the production of larger droplets, can still be the "Achilles heel" of spray tower operation, because it is the only part of the operation where gas flow must be somewhat restricted. These limitations and the fact that the spray and venturi apparatus each offers advantages not shared by both, have given rise to the development of combination gas reacting devices. These combination arrangements generally combine the features of venturi and spray apparatus into one module. These recent designs offer greater performance, allowing high removal efficiency of both gaseous pollutants such as SO.sub.2 and particulate matter such as fly ash, but at a very high cost. It is, therefore, desirable to provide an improved gas reacting apparatus which combines all of the advantages offered by venturis and spray towers into one apparatus.